1. Field of the Invention
The present invention relates to a solar cell module, particularly to a solar cell module which has a light-introducing side (incident light face) coated with a resin.
2. Relating Background Art
In recent years, environmental pollution has become a worldwide concern. In particular, global temperature rise caused by the greenhouse effect of emitted carbon dioxide is a serious problem. Therefore, clean energy generation without using fossil fuels is strongly desired. The solar cell, which is a photoelectric converter element, is promising as a clean energy source at the present time because of its safety and ease of handling.
The known solar cells include various types, typically exemplified by crystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells (including microcrystal cells), copper indium selenide solar cells, other compound semiconductor solar cells, and the like. Of these, polycrystalline silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells are being actively studied since these types of solar cells can readily be produced in a large-area form at a low cost.
Further, of these solar cells, thin film solar cells typified by amorphous silicon type solar cells, which are produced by depositing amorphous silicon on an electroconductive metal substrate and forming thereon a transparent electroconductive layer, are promising for future use as a module because of the light weight, the high impact strength, and the high flexibility thereof. However, this type of module needs to have the light-introducing face covered with a transparent material to protect the solar cell, differently from the one produced by depositing silicon on a glass substrate. Usually, glass or transparent fluororesins such as a fluororesin film, a fluororesin coating and the like have been used as the outermost coating material. Beneath the outermost covering layer, various transparent thermoplastic organic resins may be used as a filler. Glass is employed as the outermost layer because of its high weatherability which maintains the light transmissivity and thus does not cause a reduction in the conversion efficiency of the solar cell module. On the other hand, the fluororesin is employed as the outermost layer because of its high weatherability and high water-repellency which maintain the light transmissivity and thus do not cause a reduction of the conversion efficiency of the solar cell module resulting from deterioration or soiling of the surface, and further because of the ease of the sealing of the solar cell module by heat treatment. A thermoplastic resin is employed as the filler because it is inexpensive when used in a large amount for protection of the inner contained photovoltaic element.
FIG. 4 shows an example of such a conventional solar cell module. In FIG. 4, the module is constituted of a fluororesin layer 401, a transparent thermoplastic organic resin layer 402, a photovoltaic element 403, and an insulation layer 404. In this example, the same organic resin used for the light-receiving face is used for the back face insulation layer. More specifically, the fluororesin film 401 is exemplified by a film of ETFE (ethylene-tetrafluoroethylene copolymer), and a film of PVF (polyvinyl fluoride). The transparent thermoplastic organic resin 402 is exemplified by EVA (ethylene-vinyl acetate copolymer), and butyral resins. As the insulation layer 404, various organic resin films such as a nylon film, an aluminum-laminated Tedlar film, and the like may be used. In this example, the transparent thermoplastic organic resin 402 serves as an adhesive for bonding between the photovoltaic element 403 and the fluororesin 401, and between the photovoltaic element 403 and the insulation layer 404 if present therebetween, and also serves as a filler for protecting the solar cell from external scratching and impact.
No material is known, however, which simultaneously gives weatherability and impact strength, in the aforementioned surface coating constitution, especially in the case where the solar cell is exposed to a natural environment for a long time, e.g. twenty years or more. Specifically, a glass sheet as the outermost surface layer is liable to be broken by hailstorm or impact by a small stone and thus fail to protect the photovoltaic member. On the other hand, a fluororesin as the outermost surface layer loses weatherability owing to loss of stabilizers contained therein by decomposition by UV light, water or heat, by volatilization or elution by heat or water for a long term of outdoor exposure of twenty years or more, resulting in deterioration of the solar cell. Generally, resins become colored under action of ultraviolet light, ozone, nitrogen oxides, or heat. In particular, tandem junction laminated photoactive semiconductor layers, for which a non-monocrystalline semiconductor, preferably an amorphous silicon semiconductor is used, are greatly adversely affected in conversion efficiency by discoloration of surface coating material. More specifically, a tandem laminated photovoltaic member generates electricity in each of the laminated photoactive semiconductor layers respectively at different wavelengths of light. Therefore, if shorter wavelengths of light are absorbed by discolored surface coating material, the photoactive semiconductor layer absorbing the shorter wavelengths of light generates less electric current, and consequently the other photoactive semiconductor layers operate under current-limiting conditions to greatly lower the overall conversion efficiency of the photovoltaic member.
When the surface layer is a fluororesin, the resin stabilizer volatilizes to lower the conversion efficiency significantly in comparison with a glass surface layer. A resin photo stabilizer is described in a report of JPL (US Jet Promotion Laboratory) entitled "Department of Energy Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants", Annual Report, June, 1979. Specifically, the stabilizer is bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate. However, such a secondary amine type of photo stabilizer does not improve sufficiently the photostability and heat resistance of the resin. The above problem is more significant in solar cell modules having no cooling means and in modules integrated with building materials such as a roof exposed to high temperatures. At a module temperature of 80.degree. C. or higher, the surface coating material of the module is discolored at a higher discoloring rate.
The manufacturing process of a solar cell module generally includes a heat bonding step at about 130.degree. to 160.degree. C. for about 20 to 100 minutes. The heat bonding conditions depend on the crosslinking degree of the resins such as EVA which are fillers and adhesives. This heat bonding step can be accelerated by raising the temperature to shorten the treatment time. However, the high temperature will cause slight discoloration of the module after the heat bonding to disadvantageously decrease the initial conversion efficiency. One method for offsetting such a disadvantage may be selection of a cross linking agent, namely an organic peroxide having a lower decomposition temperature, to a be mixed in the filler resin: for example, selection of an organic peroxide having a radical-generating temperature lower by about 20.degree. C. However, use of a low-temperature crosslinking filler results in insufficient bonding strength of the component members with the filler, although the discoloration of the resin after the heat bonding can be prevented. Solar cell modules designed for roof use, which are required to have good appearance, are designed to be reinforced with a colored steel plate. The colored steel plate is produced generally by use of a silicone type leveling agent for the purpose of preventing pin-hole formation on coating application and preventing soiling. For bonding to such a coated steel plate, sufficient bonding strength can be achieved only by a radical abstraction reaction using an organic peroxide. The aforementioned low-temperature-crosslinked filler, which can generate radicals, does not provide sufficient covalent bonds because of the low temperature of the member. Accordingly, there is a problem that the bonding strength in this case is lower than that by heat-bonding at a high temperature.